greybus: loopback: update loopback operation description comment

Old comment needs updating to match 8a282c411ba0 ('greybus/loopback: make
loopback type input equivalent to protocol type')

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>